Optical fiber connectors and splices (hereinafter referred to collectively as "connectors") are an essential part of substantially any optical fiber communication system. For instance, connectors may be used to join segments of fiber into long lengths, or to connect fiber to active devices such as radiation sources, detectors or repeaters, or to passive devices such as switches or attenuators.
One commercially significant connector type can be generically described as comprising first and second tubular means (to be referred to herein as first and second "plugs") adapted for receiving in the bore thereof an end of an optical fiber, and means for maintaining the first and second plugs radially aligned such that the cores of the respective optical fibers in the plugs are substantially aligned. Connectors of this type will be referred to herein as "aligned plug" (AP) connectors. AP connectors typically comprise means for maintaining the fiber ends longitudinally essentially fixed relative to each other. Typically, the first and second plugs are cylindrical, with the axis of the longitudinal bore substantially coinciding with the axis of the cylindrical plug. Furthermore, the means for maintaining the plugs radially aligned are in contact with the outer cylinder surfaces of the respective plugs, and the connector is without provision for varying the radial position of the optical fiber core relative to the axis of the plug wherein the fiber is held.
Examples of such connectors are disclosed, for instance, in U.S. Pat. No. 4,541,685; U.S. patent application Ser. No. 527,341, filed Aug. 29, 1983, by T. D. Mathis and C. M. Miller; U.S. Pat. No. 4,545,644; and U.S. patent application, Ser. No. 795,483, filed Nov. 6, 1985, by J. A. Aberson, Jr. et al., all co-assigned with this.
The '685 patent discloses a connector comprising two cylindrical plugs and an alignment sleeve. AP connectors of this type, in which the alignment means circumferentially surround, or at least substantially surround, the two plugs, will be referred to as "plug-and-sleeve" connectors. The '341 patent application teaches, inter alia that glass plugs can advantageously be used in plug-and-sleeve connectors. The '644 patent discloses a connector wherein the alignment means comprise cylindrical alignment rods, with typically two of the alignment rods carrying a "flat" extending from one end of the rod to approximately the middle of the rod, whereby exact fiber core alignment can be achieved by rotation of one plug relative to the other. The alignment rods are maintained in contact with the plugs by appropriate means such as a substantially triangular flexible fixture. The '483 patent application teaches that "contiguous" glass plugs can be advantageously used in connectors of the plug-and-sleeve type, where by "contiguous" plugs are meant plugs that have occupied adjacent positions in the glass preform rod from which they are cut. The use of contiguous plugs reduces or eliminates differences in the eccentricity of the bore relative to the cylinder surface of the plugs, resulting in improved fiber core alignment. The '483 application also discloses a "corrugated tube" alignment sleeve that can advantageously be used in plug and sleeve-type connectors.
AP connectors typically require the use of two plugs of essentially identical diameter to obtain a stable low loss connection, and to insure approximately equal and adequate holding force on both plugs. For instance, if in a prior art plug-and-sleeve type connector, such as shown in the '685 patent, plugs that differ significantly in outside diameter are used, the sleeve cannot properly grip the smaller plug, resulting, inter alia, in possible tilt or other misalignment of the fiber cores as well as in possibly inadequate retaining force on the smaller plug. This occurs because the two halves of the sleeve are tightly coupled with respect to radial deformation. By this we mean that radial extension of one sleeve half, e.g., by insertion of a plug into the sleeve, leads to an analogous radial extension (of the same type, and of similar but not necessarily of exactly the same magnitude) of the other sleeve half.
Reduced mechanical and thermal stability of the connection as well as, typically, increased optical loss would result also in other AP connectors if plugs of substantially different outer diameter were mated. For instance, in one commercially available AP single mode fiber connector that uses glass plugs and glass alignment rods, specifications call for the outside diameters of the two mating plugs to be equal to within about 1 .mu.m. Maintenance of such close tolerances involves manufacturing costs that would not be present if this specification could be relaxed.
At least in some prior art AP connectors, e.g., in plug-and-sleeve type connectors, it not only is necessary that the pair of plugs be closely matched with respect to their outside diameter but also that the diameter of the matched plugs be kept within relatively narrow limits, in order to achieve stable optical coupling. For instance, in one commercially available plug-and-sleeve type connector (of construction similar to that disclosed in the above referred to '341 patent application) the permitted variation in plug diameter is about .+-.6 .mu.m. Use of plugs with diameters outside this range (even if the two plugs are matched) results in relatively poor gripping and/or alignment of the plugs by the sleeve, or overstressing of the sleeve, and, consequently, in unstable mechanical and optical coupling between the fibers.
As will undoubtedly be appreciated by those skilled in the art, a piece part having one or more dimensions with close tolerances is more costly to manufacture than it would be to manufacture the same part with larger tolerances. Thus, significant cost reduction could be attained with alignment sleeves capable of accommodating a greater range of plug diameter.
In view of the economic advantages and possible improved performance that would result if current tight tolerances on the outside diameters of plugs to be used in AP connectors could be relaxed, AP fiber connectors able to accommodate plug pairs whose members differ in outside diameter by more than is acceptable in analogous prior art connectors, and/or that can accommodate a substantially greater range of plug diameters than is possible with prior art AP connectors, would be of substantial interest. This application discloses such connectors.